The slaughter in San Bernardino, California, in some ways does not fit the mold of past mass shootings. Details that have emerged about the massacre contrast with other examples of a form of violence typically carried out by men acting alone.

Some of the elements that make the latest attack especially unusual:

FEMALE SHOOTER

One of the two assailants, Tashfeen Malik, 27, was a woman. She and her husband or fiance, Syed Rizwan Farook, left behind their 6-month-old daughter before carrying out the rampage and later dying in a shootout with police. Grant Duwe, a criminologist with the Minnesota Department of Corrections, said the attack was only the third out of 166 public mass shootings involving four or more fatalities over the last 100 years to involve a female shooter. The others involved Cherie Lash Rhoades, charged with killing four in a California tribal shooting last year, and Jennifer San Marco, who fatally shot six postal workers in Goleta, California, and killed herself in 2006.

___

MULTIPLE ASSAILANTS

While rumors often circulate about a second shooter, they nearly always are proven false. In this case, Farook and Malik carried out the attack together with assault rifles and semi-automatic handguns. Adam Lankford, a criminal justice professor at the University of Alabama, said his review of mass public shootings indicates it is the first in recent history to involve a joint male-female team of shooters. Duwe said it was also the first public mass shooting to involve multiple assailants since a pair of teenagers killed 12 fellow students and a teacher at Columbine High School.

___

POSSIBLE MIXED MOTIVE

Mass shootings are typically carried out as a kind of revenge by disturbed individuals who feel they have been wronged. In San Bernardino, officials have raised possible motives including extremism, a workplace dispute or some combination of factors. Criminologists say there are some prior examples of shooters who identified with extremist ideology but also were dealing with turmoil in their personal lives. Lankford said that appeared to be the case for shooters including the 2009 attack at Fort Hood in Texas; the shooting last summer at a military recruiting center in Chattanooga, Tennessee; and a 1997 shooting at New York's Empire State Building.